As one important type of flat plate display, a LCD, such as TFT LCD, has been developing rapidly in the last decade and has attracted much attention. Due to the severe competition among the manufactures and advancement in manufacturing technology of TFT LCD, LCDs with excellent display performance and lower price have been increasingly put into the market. Therefore, introduction of more advanced manufacturing technology to further simplify the production process and reduce the production cost has become more important to enable the manufacturer to survive in the severe competition.
The manufacturing technology for TFT LCD array substrate has undergone the map from the seven mask (7Mask, one cycle of photolithography is processed with one mask) technology to the current five mask (5Mask) technology, and the 5Mask technology today has become the mainstream for manufacturing TFT LCD array substrate.
Some manufacturers have attempted to exploit the four mask (4Mask) technology in fabrication. This 4Mask technology is based on the previous 5Mask technology, in which the mask for forming active layer (Active Mask) and the mask for forming source/drain electrode (S/D Mask) are merged into a single one with the aid of a gray tone mask, and the function of the original two masks, i.e., Active Mask and S/D Mask, is achieved by the single mask through the modification to the etching processes.
A gray tone mask has a slit-shaped pattern thereon, and besides the fully transparent and opaque regions, partially transparent patterned regions are formed due to the interference and diffraction of the light passing the patterned regions of the mask. During exposure, the light only partially passes through the partially transparent portions. By controlling the exposure quantity, the light passing through the partially transparent portions illuminates portions of photoresist and has the portions partially exposed, and the light passing through the fully transparent portions of the mask illuminates the other portions of the photoresist and has these portions fully exposed. After development, no photoresist exists in the fully exposed regions, and photoresist thickness in the partially exposed regions is less than that in the non-exposed regions, so that the exposed photoresist is shaped in three-dimension. The photoresist thickness after development can be controlled by controlling the transmittance ratio among the respective regions of the gray tone mask, i.e. the “duty ratio” of the slit region to the empty region. The method of forming a three-dimensional pattern with different thickness in the photoresist through a mask with partially transparent patterned region can be collectively called gray tone mask technology.
Referring to FIGS. 1 and 2, the conventional 5Mask technology uses five masks for photolithography, including the masks for forming a gate electrode a (Gate Mask), an active layer b (Active Mask), a source/drain electrode c (S/D Mask), a via hole d (Via Hole Mask), and a pixel electrode e (Pixel Mask), respectively. Each process using the respective masks further includes one or more thin film deposition, photolithography with the mask and etching (e.g., dry etching or wet etching) processes, thus resulting in five cycles of thin film deposition, photolithography, and etching processes, as shown in FIG. 2.
A typical pixel unit of the TFT LCD array substrate manufactured by the above conventional 5Mask technology is shown in FIG. 1.
Compared with the existing 7Mask technology, the 5Mask or 4Mask technologies has greatly simplified the manufacturing processes and significantly improved the utilization ratio of the equipment and the production capacity, but such technologies still suffer from the drawbacks such as complicated manufacturing process, low production capacity and low utilization ratio of the equipment.